Regulation of Protein Traffic in Polarized Epithelial Cells: The Polymeric Immunoglobulin Receptor Model

Most eukaryotic cells are spatially asymmetric or polarized. One of the most basic types of polarized cell organization found in multicellular animals is the epithelial cell (for review, see Mostov et al. 1992; Rodriguez-Boulan and Powell 1992; Mostov and Cardone 1995). Epithelial cells form sheets that line cavities, or they externally cover an organism. In this manner, these cells constitute the fundamental interface between the organism and the outside world. A simple epithelium is formed by a monolayer of cells. A circumferential tight junction divides the plasma membrane of each cell into two domains: The apical plasma membrane faces the lumen of the cavity, and the basolateral plasma membrane faces adjoining cells and the underlying basement membrane and connective tissue. These two domains of the plasma membrane differ in their protein and lipid compositions, reflecting the different functions served by the two surfaces in a polarized cell.